The present invention relates to a clamping mechanism for securing a semiconductor wafer during wafer handling. More particularly, the present invention is directed to a four bar linkage mechanism that securely clamps a semiconductor wafer near the distal end of a robot arm.
A wafer is the base material, usually silicon, used in semiconductor chip or integrated circuit fabrication. Typically, the wafer is a thin slice of base material cut from an ingot or xe2x80x9cboule.xe2x80x9d Each 8 inch (200 mm) production wafer is approximately {fraction (1/30)} inches (0.85 mm) thick and has a diameter that varies by xc2x11 mm. Because of the nature of the base material and the thinness of each slice, the wafer can easily be damaged through mishandling.
Wafers are typically processed into semiconductor chips by sequentially exposing each wafer to a number of individual processes, such as photo masking, etching and implantation. Modem semiconductor processing systems include cluster tools that aggregate multiple chambers together, where one or more of the individual processes are performed in each chamber. These chambers may include, for example, degas chambers, substrate pre-conditioning chambers, cool down chambers, transfer chambers, chemical vapor deposition chambers, physical vapor deposition chambers, etch chambers, or the like.
Typically, these chambers surround a central chamber housing a wafer handling robot. The cluster tool also typically includes a cassette in which multiple wafers are stacked before and after semiconductor fabrication. The wafer handling robot has access to the multiple chambers and the cassette, through ports coupling each chamber and cassette to the central chamber. During operation the wafer handling robot repetitively transports wafers from one chamber to another, or to and from the cassette. Furthermore, the cluster tool forms a sealed environment that is controlled to limit potential contamination of the semiconductors and to ensure that optimal processing conditions are maintained. Examples of cluster tools can be found in U.S. Pat. Nos. 5,955,858, 5,447,409, and 5,469,035, all of which are incorporated herein by reference.
To increase fabrication efficiency, a high throughput of wafers is desirable. A high throughput can be achieved in a number of ways. First, duplicate chambers can be provided. This potential solution, however, substantially increases the cost and complexity of each cluster tool. Second, additional wafer handling robots can be provided in each cluster tool. Again, this solution substantially increases the cost and complexity of each cluster tool. Third, the speed of any individual process can be increased. Although optimization of each process is always being improved upon, each process is typically completed in as short a time as is currently possible. Finally, the handling speed of each wafer by the wafer handling robot can be increased. This solution, however, is subject to a number of criteria, such as: each wafer must be securely grasped or clamped by the wafer handling robot in the minimum amount of time; the clamping of the wafer must be firm, but not overly so, so as not to damage the fragile wafer; the clamping and placement of each wafer must be precise and accurate since any misplacement might negatively impact the process and/or damage the wafer; transfer between chambers, or into or out of the cassette, must be smooth so that the wafer does not undergo any unnecessary stress and in the worst case, if the wafer is dislodged from the clamping mechanism, this condition must be sensed, and the wafer transfer system must be halted; the clamping mechanism must be heat resistant, as some of the processes may expose the clamping mechanism to high temperatures; the clamping mechanism must not introduce any particulates or contaminants into the closed environment that can ultimately damage the wafer or semiconductors (it has been found that particulates as small as the critical dimension or line width of a semiconductor device, currently 0.18 xcexcm, can damage the integrity of an integrated circuit formed on a wafer); the wafer clamping mechanism should be able to automatically center a misplaced wafer; and finally, the wafer clamping mechanism must not introduce a static electric field into the wafer, which might discharge and damage the semiconductor devices being fabricated.
To maximize system throughput, the wafer handling robot must rotate and extend as fast as possible without causing the clamped wafer to slip during transport. Slip occurs when the robot accelerates the wafer such that its inertia overcomes the clamping force of the clamping mechanism. This causes undesired wafer movement and results in wafer misalignment and particle generation.
Of the abovementioned potential solutions to increasing wafer throughput, increasing the handling speed of each wafer is the most practical and cost effective. Therefore, to address the above criteria, a more robust and better designed wafer clamping mechanism is required.
A number of prior art devices have attempted to clamp the wafer in a way that addresses some or all of the abovementioned criteria. FIGS. 1A, 1B and U.S. Pat. No. 5,955,858, show a bottom view of a wrist assembly 102 of one such prior art device 100 with its bottom cover plate removed. Clamp fingers 108, shown extended from the wrist assembly 102, engage a perimeter of a wafer 104 to clamp the wafer 104 onto a wafer carrying blade 106. The wafer 104 is held between the fingers 108 and a blade bridge 110 under forces applied by a pair of parallelogram springs 112, best seen in FIG. 1B. Parallelogram springs 112 bias the fingers 108 toward the wafer 104.
The wrist assembly 102 is coupled to the distal end of frog-leg type robot arms 114 of a wafer handling robot. During extension of the robot arms 114, i.e., when the robot arms are drawn toward one another, as shown in FIG. 1A, a rotation is imparted on pivots 116, which in turn rotate cogs 118. The cogs 118 in turn engage with the fingers 108 to retract the fingers 108 away from the wafer 104. Therefore, the wafer 104 is released when the robot arms 114 are extended and clamped when the robot arms 114 are retracted. If the fingers were directly attached to the cogs 118 then the clamping force would depend on the motion characteristics of the robot, for example, the speed of extension and retraction of the robot arms 114. In this device, the fingers can be set independently by controlling the stiffness of the parallelogram spring 112.
A drawback of this wrist assembly 102 is that the parallelogram springs 112 are easily deformed by out-of-plane forces, causing the clamping force direction to deviate from the norm. This leads to unreliable clamping and potential particle contamination caused by friction between the fingers and the wafer. Furthermore, the cycle life of the parallelogram springs 112 (approximately 1 year or 10 million spring cycles) has been found to be inadequate. In addition, the wrist assembly 102 does not provide for clamping a wafer that is not centered correctly. If the spring is deformed, the capture pocket, i.e., the total area in which the clamping mechanism can capture a wafer, could easily change, thereby, reducing the tolerance of the wafer handling system to deviations in the position of the wafer during transfer to and from each chamber. It has also been found that manufactured parallelogram springs are highly sensitive to manufacturing defects and mishandling before, during, and after installation, leading to unreliable clamping. Furthermore, the manufacturing process for the spring requires an electropolish step that cannot be controlled reliably. Finally, any kinks in the spring caused by mishandling lead to stress concentration points that reduce the fatigue life of the spring.
A partial bottom view of another prior art clamp wrist assembly 102 with its bottom cover plate partially removed is shown in FIG. 1C and in U.S. Pat. No. 6,155,773. This clamp wrist assembly 120 comprises a lever assembly 122, a flexure member 124, and a pair of clamp fingers 126 that engage a wafer 130. Leaf springs 128 bias the flexure member 124 against the wafer 130. When the clamp wrist assembly 120 is in its extended position, a translational member 132 engages a first lever 134 to retract the fingers from their clamping position. However, this wrist assembly 120 does not clamp a wafer that is not centered correctly. Moreover, space limitations prevent this clamp wrist assembly 120 from being implemented on an opposed dual blade robot.
Finally, a partial bottom view of another prior art wafer holder 140 with its bottom cover plate removed is shown in FIG. 1D and U.S. Pat. No. 5,810,935. Wafer holder 140 includes holding means 142 for holding rounded edges of wafer 144, and an actuating means 146 for operating the holding means 142. Tension springs 148 bias the holding means 142 towards the wafer 144. Not only does the actuating means introduce additional complexity and cost into the system, but it leads to more potential areas of particle generation and potential electrical fields, both of which might damage the wafer.
In light of the above, there is a need for a wafer clamping mechanism that securely clamps a wafer for speedy handling, meets the abovementioned criteria, and addresses the drawbacks presented by the prior art.
A preferred embodiment of wafer clamping mechanism of the present invention comprises a linkage mechanism and a wafer contact point coupled to the linkage mechanism. The linkage mechanism is preferably coupled near to the distal end of a robot arm. The linkage mechanism preferably comprises a four-bar linkage having: a first link having a first fixed pivot and a first floating pivot distal from the first fixed pivot; a second link having a second fixed pivot and a second floating pivot distal from the second fixed pivot; and a third link having a first coupling pivot rotatably coupled to the fist floating pivot, and having a second coupling pivot rotatably coupled to the second floating pivot. In use motion of the robot arm activates the linkage mechanism, which in turn causes the wafer contact point to clamp a wafer.
Therefore, the above described clamping mechanism reliably increases throughput while reducing cost. The clamping mechanism also provides the benefit of passive wafer centering, versus more costly active center finding methods, thereby eliminating the potential for failure due to variances in wafer placement.